The present invention relates to a circuit for detecting the shutoff of an optical output for use particularly in circuitry for sending optical signals in bursts or cells.
In parallel with the progress of information-oriented society, communication technologies using optical fibers are under development to implement economical, ultrahigh speed digital communication. Japanese Patent Laid-Open Publication No. 2-290341, for example, discloses a bidirectional optical transmission system capable of detecting the shutoff of a light beam for protecting the eyes of an operator working at an office from a light beam output from an optical transmitter. Japanese Patent Laid-Open Publication No. 10-256990 teaches a simple optical transmitter/receiver constructed to constantly monitor a stand-by system by using the same optical wavelength band as a main signal. Further, Japanese Patent Publication No. 6-83153 proposes a fault monitoring method allowing the individual subscriber station to detect a fault occurred on the subscriber line with the respective monitoring mechanism.
The conventional transmission of optical signals in the form of bursts or cells will be briefly described hereinafter. FIG. 1 schematically shows a conventional PON (Passive Optical Network) system including three ONUs (Optical Network Units) 111, 113 and 115 each being situated at a particular subscriber station. The ONUs 111, 113 and 115 are connected to an office 117 via a star coupler 119.
In the PON system shown in FIG. 1, optical signals output from the ONUs 111, 113 and 115 are multiplexed on a time division basis and sent toward the office 117 (up-going direction hereinafter) in the form of cells. A transmitter/receiver, not shown, built in each of the ONUs 111, 113 and 115 must include a circuit for monitoring the respective optical signal to see if a normal optical output level is being sent or not, i.e., to detect the shutoff of the optical output.
Reference will be made to FIG. 2 for describing an optical output shutoff detecting circuit included in each of the ONUs 111, 113 and 115. As shown, the circuit is generally made up of a laser diode (LD) 101, a photodiode (PD) 102, a preamplifier 103, an amplifier 104, a peak detector 105, a comparator 106, and a driver 107.
A clock signal Sclk and a data signal Sdata are input to the driver 107. The driver 107 digitally modulates the data signal Sdata in synchronism with the clock signal Sclk and drives the LD 101 with the resulting modulated signal. The PD 102 monitors part of a light beam issuing from the LD 101 while generating a photocurrent.
The preamplifier 103 coverts the above photocurrent to a voltage to thereby output a voltage signal. The amplifier 104 further amplifies the voltage signal output from the preamplifier 103. The peak detector 105 detects the peak value of the voltage signal output from the amplifier 104. The comparator 106 compares the detected peak value with a reference voltage Vref in order to determine whether or not the optical output level is higher than a preselected level.
FIG. 3 shows waveforms representative of specific outputs of the constituents of the above shutoff detecting circuit. As shown, the preamplifier 103 outputs a signal Spd while the amplifier 104 outputs a signal Samp by amplifying the signal Spd. The peak detector 105 searches for the peak value of the signal Samp, as represented by a signal Speak. The comparator 106 outputs a signal Salm representative of the result of comparison. Specifically, the comparator 106 outputs a high level (high voltage) when the signal Speak is lower than the reference voltage Vref or outputs a low level (low voltage) when the former is lower than the latter. The low level of the signal Salm indicates that the optical output level of the LD 101 is lower than a preselected level (generation of an alarm).
With the above configuration, the conventional circuit shown in FIG. 2 is capable of easily detecting the shutoff of the optical output on the basis of the signal Salm output from the comparator 106. However, the conventional circuit is not always successful to detect the shutoff of the optical output. Specifically, the peak detector 105 relies on the charging and discharging characteristic of a capacitor. While a period of time of the order of nanoseconds (Tchg, FIG. 3) suffices for the capacitor to be charged for detecting a peak value, a period of time of the order of more than microseconds (Tleak) is necessary for the capacitor to be discharged due to free discharge. In a system with a transmission rate of, e.g., 156 Mb/s in the up-going direction, the above period of time Tleak corresponds to as many as 100 bits.
The problem of the conventional shutoff detecting circuit will be described more specifically with reference to FIGS. 4A and 4B. There are shown in FIGS. 4A and 4B specific optical output monitor signals Spd and specific shutoff alarm signals Salm appearing in the circuit of any one of the ONUs. In FIGS. 4A and 4B, cells #2, #3 and so forth sequentially appear. First, as shown in FIG. 4A, so long as the interval Tsp between the consecutive cells is sufficiently longer than the capacitor discharge time Tleak, the signal Salm successfully goes high before the next cell appears. However, as shown in FIG. 4B, assume that the interval Tsp is shorter than the discharge time Tleak. Then, when the shutoff of the optical output occurs in the middle of, e.g., a cell #4, the circuit fails to detect it immediately and erroneously determines that the next cell #5 also has the expected optical output level.
Particularly, in a system capable of freely varying the band allotment to ONUs, i.e., the occupancy ratio of the cells of the individual ONU in the up-going time domain, it frequently occurs that the cells of a particular ONU continuously appear, as shown in FIG. 4B. This aggravates the above erroneous detection by the conventional circuit.
It is therefore an object of the present invention to provide an optical output shutoff detecting circuit capable of automatically detecting the drop of an optical output level with high accuracy and generating an alarm immediately.
An optical output shutoff detecting circuit of the present invention includes a flip-flop. The flip-flop has a D terminal to which a signal representative of a result of comparison between the output level of an optical signal output from a laser diode and a reference signal is input, and a C terminal to which a signal produced by delaying a data signal is input. The output signal of the flip-flop is used to detect the output level of the optical signal.
The flip-flop included in the above circuit determines an emission state every bit of a data signal. It follows that when the optical output level drops below a preselected value, the circuit can detect the drop immediately.
Further, the circuit may include an AND gate for ANDing the output of the flip-flop and a signal representative of the duration of a cell to be sent in order to enhance reliable detection.